Pyrometallurgical process for the production of pig-iron and ferrochromium



Patented June 16, 1942 PRODUCTION OF PIG-IRON MIUM CHRO

AND mano- Percy H. .itoyster, Montclair, N. J.

No Drawing. Original application March 9, 1939,'-

Serial No. 260,866.

Divided and this application August'5, 1941, Serial No. 405,553

4 Claims (Cl. 75-127) This invention relates to the pyrometallurgy of acompound or complex ore (hereinafter referred to as mixed ore)containing an oxygen compound of iron and at least one oxygen compoundof at least one other metal more difiicultly re-' ducible than theoxygen compound of iron but less difiicultly reducible than an oxygencompound of silicon (e. g., less diificultly reducible than S102). Theinvention is concerned with an improved process of smelting such mixedore whereby to produce a ferrous alloy product having a relatively highcontent of said other metal. The improved process has particularapplicability to the production, from low grade chrome ore (by "lowgrade here being meant an ore containing a larger ratio of iron to said"other metal than is desired to be present in the alloy product, andusually containing a substantial amount of gangue) of ferro-chromium ofat least 30% grade, and iron.

The invention is concerned, also, with novel steps of the completeprocess.

In accordance with the general process of the present invention Isubject the aforesaid mixed ore to a preliminary metallurgicaltreatment, by which treatment a substantial (e. g., major) portion ofthe iron content of the ore is reduced and. separated out as a moltenprimary metal and a major portion, or substantially all, of the contentof the othermetal is incorporated in nonmetallized form in a primaryslag, whereupon said primary slag, in the second step of the process, issmelted to produce a secondary slag of low metal content and a secondarymetal containing the greater part of the charges content of said othermetal. Thus, I may, in the second step; smelt in the blast furnace apre-fused material resulting from a prior treatment of a mixed ore, bywhich treatment a major or at least substantial portion of the ironcontent of the mixed ore had been differentially or selectively smeltedand separated, as liquid metal, from the concurrently produced liquidslag containing in nonmetallized form the major portion of the contentof other metal of the original ore. Differently expressed, I may subjectthe mixed ore to difi'erential or selective reduction of a major portionof the iron content thereof in a blast furnace operation, therebyproducing a slag havinga ma-w terially lowerv ratio of iron to othermetal" than had the original mixed ore, and thereafter metallize theother metal" (and also the residual iron content) in a smeltingoperation.

In carrying out the complete process according to the preferredembodiment thereof, in the preliminary metallurgical treatment I chargethe mixed ore and solid carbonaceous fuel (e. g., coke) into a blastfurnace, blow the charge with a preheated air blast, and maintain in thefurnace conditions favorable to effect metallization of the major partof the iron content of the ore while retaining in the molten slag mostof the content of the other metal in the form of an oxidic compound. Incarrying out the second step I blow the charge with an air blastmaintained at atemperature of 1200 F., or higher, so as to provide inthe furnace hearth a suflicient' amount of heat of high availability forreduction not only of the residual oxygen compound of iron but also ofthe more dimcultly reducible oxygen compound present in the slag beingtreated.

In applying the hereinbefore generally described process to theproduction of ferro-chromium from a chrome ore, the first step is socarried out as to effect dissolution of metal values of the ore in asuitable mineral solvent, of relatively low melting point and lowviscosity, comprising molten common refractory oxides so selected as toyield an acid solution of the ore. This solution is then smelted to aferro-chromium alloy product. a

The process of the present invention, to be described more particularlyin the following paragraphs, is concerned not only with the combinationof a step of preliminarily treating a chrome ore for the purpose ofrendering the same more readily smeltable with the step of smelting theso treated ore, but also with the separate steps and with the product ofthe preliminary treatment step.

In my copending patent application Serial No. 252,448, filed January 23,1939, (now U. S. Patent No. 2,238,078) there is described a process bywhich a material (specifically an ore) containing chromium oxide may besmelted in a blast furnace. It is there pointed out that the majordifliculty in such a process is occasioned not by any'na'turalirreductibilityf of the compound CrzOs (Whether this oxide is present inisolated fit form, or liquid, or in solution as such or in the form of aferrous chromite," FeO.CR,2O3, calcium chromite, CaO.C!2Oa, or otherspinels) but rather by its highly refractive nature. Chromic oxide isreadily reducible by solid carbon, at temperatures below 2000 F.,according to the equation but the two reactants cannot be brought intochemically reactive contact unless the CrzO3 is in liquid form (moltenor in solution), since progress of the reaction of Equation 1 from leftto right is dependent upon bringing the carbon atoms within moleculardistances (e. g., cm.) of the oxide molecule. Unfortunately, the meltingpoint of CI203 is high (4130 F.). The compound is insoluble in SiOz; itforms only solid solutions with MgO (minimum temperature 4130" F.) andwith A1203 (minimum temperature 3722 F.); and in the CaO.CrzO3 binarysystem it forms the single extremely refractory compound CaQCrzOa M. P.4000 F. The two eutectics in this latter binary system are each highchrome ores, by a process involving as a first step a preliminarytreatment of said ore in a blast furnace whereby to bring the chromiumoxide content of the same into liquid (and, therefore, smeltable) form,with subsequent smelting of the so-treated ore in a second step of thecomplete process. By this procedure essentially the same final result isattained that is attained by the carrying out of the process described.and claimed in the aforesaid copending application, with the advantagesthat the thermal requirements of temperature points '(3430 F., at 51%CaO and F. (i. e., the eutectic between Fe0.CrzOs and Cl'zOs at 23% FeOand 77% CrzOs). Chromites of the exact composition FeO.Cr2O3 are ofinfrequent occurrence: in natural chrome ores a por-- tion of the FeO isoften replaced by MgO, and a portion of the CraOa is often replaced byA1203 and Fe'zOz. Silica present in the ore occurs generally as freequartz or else as magnesium orthosilicate (forsterite, Mg2SiO4, M. P.3866" F.). Magnesium aluminate (spinel) is a common contaminant. It willbe seen, therefore, that a chrome ore contains CIzOs accompanied by thegangue oxides MgO, A1203 and SiOz: it is highly refractory, and is notreadily fluxed by CaO, which latter is also highly refractory (M. P.4680 F.).

As has been described in my copending application above identified, Ihave discovered that chrome ore can be smelted in the blast furnaceprovided the blast temperatureemployed exceeds a specific minimum, witha resultant elevation of the temperature of the hearth (metal and slagbaths), above conventional blast furnace practice, and more particularlywith a substantial and necessary increase in the temperature of thecombustion zone gases, in the tuyere breast and bosh of the furnace,above the present-day operating level. It is there disclosed that suchelevation of hearth and bosh temperatures above their standard levelscould successfully be accomplished by raising the temperature of the hotblast, employed in the smelting, from the prevailing range (l200l600 F.)of present-day pig iron and ferromanganese blast furnace practice into acompletely new temperature range of from 1600 to 3000 F. Such high blasttemperatures are not attainable with blast heating equipment currentlyused by domestic or foreign blast furnace plants, but are readilyattainable by means of the so-called pebble-type stoves described in U.S. Reissue Patent No. 19,757.

The process of the present invention consists in an improvedmetallurgical procedure for effecting economic advantages in thesmelting of that single step process may be diminished and the necessityof maintaining the upper extremities of bosh and hearth temperature maymaterially be ameliorated, that lower hearth temperatures may betolerated, with a consequent improvement in thermal efficiency and infuel requirements, and that a lower blast temperature may be employed.Operation of the furnace at lower bosh and hearth temperatures resultsin conjugate benefits in the way of decreased heat loss, less severewear, less rapid destruction of the furnace brick work, and in areduction in the otherwise drastic Water cooling requirements.

In accordance with the improved process of the present invention, thedesired reduction of the CrzOs content of the chrome ore is effected byfirst dissolving the chrome ore'in a suitable mineral solvent, ofrelatively low melting point and low viscosity, consisting predominantlyof molten common refractory oxides, in an initial furnace operation inwhich a substantial reduction of the iron content of the ore iseffected, and, as a second and separate operation, smelting theresulting solution, in a suitable furnace, e. g., in a blast furnace, toform the chromium alloy ferro-chromium. The first step of the processinvolves a diminution of the chromium concentration of the initial ore,or, in other words, a preliminary dilution or depreciation of itsmetallic values. The second step consists essentially in subjecting theso-dissolved ore, now in solution in the solute, to smelting conditionsadapted to metallize the greater part, or substantially all, of themetal values of the dissolved ore. Both these steps, as well as thecombination of the two, are new and constitute the basis of the presentinvention.

This preliminary dilution of the chromium content of the ore is ofcourse contrary to, and a departure from, all previous metallurgicaloperations in the sense that, hitherto, the preliminary treatment of anyore as mined has been directed always towards the concentration of itsreducible oxide content.

The following discussion of the metallurgy involved in my process mayexplain why this novel preliminary dilution is advantageous in thesmelting of chrome ore:

Referring to Equation 1 above, it has been found that the equilibriumconstant K in the thermodynamic equation K=(Cr)COW/(Cram).(C)

is given by the equation logic K=24.97-34,000/T (3) wherein thebracketed quantities (Cr)",

(CI2O3) is the molar concentration of chrome- 2,286,577 3 oxide in theliquid oxide phase (i. e., slag bath). tion are explained andillustrated in the fol- Since "(CIQOD appears in the denominator, anylowingz.

diminution of(Cr-.-a) causes either (a) an in- Example 1 V e c e se in ep 0f reduction (b) a In the furnace treatment of ore A above I de r asein t Percentage chromium the 5 use a coke as fuel and a blast furnaceslagy- Both 01 these efiects are, D 88, here identified as -as asolvent, the fuel and s b For e p With O Phase solvent exhibiting thefollowing analyses: taining 0.500 molar concentration of CrzO; inchemical contact with a metal bath analyzing Blasmmace slagl Cr-"l5.0%;Fe--l5.90%, and Si1.00%, the ("13. F.l") molar concentration of (Cr) is0.592. In a blast furnace hearth at 14 lbs/sq. in. pressure Percent(gauge) the equilibrium constant K is 5.242 and, according to equation 3above, the reduction temperature is 2060 F. (T"=1400 K.). By dilutingthe CraOa from 0.5 to, say, 0.1 molar concentration, the reductiontemperature is raised to 2259. However, in the present case, theequilibrium temperature in the ternary system Cr--C are all so low andso easily attainable in the blast furnace that the above mentionedincrease in reduction temperature is not technically serious. Theadvantage derived from diminishing the refractoriness of the chromematerial far outweighs "the disadvantage of a moderate rise in reductiontemperature.

The solvent or carrier solution for incorporating the CrzOs maybe anyconvenient mixture of common refractory oxides so proportioned as toexhibit a minimum viscosity in the temperature range 2600 to 3000 F. andhaving a good solvent power for CrzOz. Considerations of cost restrictthe components of the mixture to the four oxides A1201, SiOa, CaO andMgO, with such other minor oxide constituents gas accompany theseoxides'in usual practice. Slag from a' blast furnace smelting iron oremay used, as may open hearth slag and/or-Bessemer- I slag as theoccasion may render desirable.' MnO, even when sufflciently cheap,cannot advantagei0 ously be used in the solvent of the present proo 1ess when one is producing high grade ferrochromium, because theferro-chromium product would in such case be contaminated h i twentyminute intervals, with rounds consisting ganese. g 'm Ores useful forthe production of chromium gz gg gg g a i g g ggz g g g fi f mtetals ofalloys ferm'chrqmmm) nace has a hearth diameter of 1'7 feet 6 inches,taming h1gh percentages of chromium, present and is blown with 29 600 cuft /min of atmos a wide variety of compositions. CH0: and FeO pheric(measuredat 0; indies of are always present and the gangue oxides MgO,5" cm 60% of humidity) at 10 lbs /sq inch pres A1203 and Sioz are seldomabsentor' negligible sure and preheated to i450" F The furnace is Lesseramounts of CaO, C02, and water of crysat hourly intervals 49 100 lbs oftallization are encountered. Phosphorus'is selchrome furnace slag" andntappdu at dom present in troublfesome amount To hour intervals, of64,500 lbs. of chromium alloy. trate a range of materials operable incarrying The analyses of the .chmme metal and of the out the embodimentsof this process, I give, in chrome furnace slag the following, thecompositions of-foursuchmaterials, designated ores A, B, C, and D:

Coke

assesses Fixed carbon.

At twenty minute intervals I charge rounds" consisting of 12,600 lbs. ofslag B.F. I and 3400 lbs. of the coke, to which I add 8400 lbs. ofchrome ore A. The furnace is blown with 11,000 cu. ft. per min., ofatmospheric air (measured at 66 F., 30 inch mercury pressure andhumidity), at a pressure of 8.5 lbs/sq. inch and preheated to atemperature of 2100 F. The 4 furnace has a hearth diameterof 14'6", andis fflushed at thirty minute intervals, discharging 39,800 lbs. of slagsolution" at the rather high temperature of 2960 F. and showing ananalysis: CaO 27.1%; MgO 7.2%; A120: 10.9%; 510: 31.2%; CrzOa 18.73%;FeO 3.22%; MnO

cu. ft. per minute of gas (60 -30 inch dry basis) I at the temperatureof 225 F., analyzing CO2 0.2%; 00 35.4%; Hz 1.24%; N: 63.1%; a portionof which gas is used in the blast heating apparatus for providing the2100 F. hot blast.

The slag discharged is permitted to crystallize and is broken, screenedand stored in bins as a charge material for use in the second furnacestep.

- In the second step I charge a blast furnace, at

Metal Chrome furnace slag Composition cm A Ore a Ore c Ore D Since thegangue oxides present in the ore, (measured at F" 30 inches of ercurypreswhen disso ved. alter the composition of the sure, dry basis) of gasat the high temperature carrier s in he carrying out of the first of1140 F., and the gas analyze." CO2 0.2%; 00 furnacing step of thismethod, it is advisable to 37 0 13 and N2 509 The temperaselect anappropriate composition for thi 5 1- ture of the slag at flush is 2885F. and of the vent with reference to the particular ore to be metal atcast is 2790, equivalent to a hearth treated. The principles governingsuch aselecis temperature of 2837 F., the latter as used here- 0.22; S0.98%. The furnace top discharges 13,650 v -'I'he furnace discharges38,000 cu. ft./min.

' in designating the average of the slag and metal I slag solution ofmuch lower grade (Cr 12.8%;

Fe 2.5%; metallics 15.3%; gangue oxides 76.6%) This slag solutioncomprises a liquid oxide solvent 'of a strongly acid nature, and in thisillustration the acid oxidediluents SiO2+Al2O3 are in excess of thebasic oxide constituents CaO-i-MgO of the solution. The molarconcentration of CrzOais low (e. g., 9%) and it is present in free oxideform. At elevated temperatures Cl'zQa does not form compounds with MgO,A120: and S102. mo and FeO are present in such small amounts that littleFeCrzOa and MnCrzO: can be formed. Any tendency of the CrzOzi, in thisdilute acid oxide slag-solution, to combine with the CaO present in theslag solution is inhibited by the conflicting mass-action tendency ofthe CaO to combine with the greater molar concentrations of S102 andA1303 present,

aaeasw I however, that by the addition to the furnace charge of amixture--preferably, prei'used-of S102 and AlzOa with 08.0 and M30 I amenabled toinaintain in uncombined oxide form the cmoa content of theslag solution, to prevent formation of any objectionable amount ofcalcium chromite, and to produce the metallurgically valuable newintermediate product described here, which product is peculiarlysuitable for smelting into term-chromium. This solution of CrzOa in aslag solvent exhibits a substantially lower melting point and a lowerviscosity than the original chrome ore.

The amount of dilution of CrzOa in the solution must, ofcourse, becontrolled within reasonable limits: its extent depends upon thecomposition of the original ore. The relative amounts of slag-solventand ore in the charge will vary in the case or the several oresencountered in pracfollowing:

the CaO present forming wollastonite (CaSiZOS) and anorthite(CaAlzShOa).

The relative proportions of the three oxide components CaO, A1203 andS102 occurring in molten blast furnace slags are such that there arevariously present four compounds:

(at) Calcium metasilicate; CaSiOz, pseudo-wol- 'lastonite, I (b) Calciumalumino-silicate; CaAhSiOa; the

feldspar anorthite, (0) Calcium alumino-silicate; Ca2Al2Si0'1,gehlenite,

, (d) Calcium 0rthosilicate;-CazSi04a very rare natural mineral.

Two of these four compounds are "neutral" or indifl'erent with respecttov basicity and acidity, viz., pseudo-wollastonite containing a singlemol each of a base (090) and of an acid (S102) and. gehlenite containing2 mols of basic oxide (2Ca0) and two mols of acid oxide (Al2O3+SiO2)However, the feldspar, anorthite, is extremely acid, containing only onemol of basic oxide (CaO) and three mols of acid oxides (A1203 plus twoS102) Anorthite is a de-chromitizing agent, the excess of acid oxides,A1203 and SiOz, tending to suppress the formation of any calciumchromite. The presence of MgO in the slag solution assists thissuppression. The following equation is believed to illustrate thisdechromatizing action:

CaCr2O4 +CaAl2Si'z0a+MgO= (Cal cium (A norfhite) chromite) 2CaSiO3+MgA12O4+ CrzOa (4) (Wollastonite) (Spinel) Example 2 With the samefurnace of Example 1, and with the same wind, the rounds are: 4000 lbs.of coke; 5900 lbs. of slag (B. F. I); and 14,000 lbs. of ore B. Theslag-solution produced has the analysis: Ca0l5.6; MgO-4.8; SiO226.4;C120s-38.5; S-0.62; FeO--2.20. The blast temperature is 1200 F. Theaccumulation of metal (mostly iron) in the furnace hearth, fromunavoidable reduction, is greater in this case (11,200 lbs. per hour)than in Example. l ore A (viz., 1380 lbs. of metal per hour), the

Example 3 With the same blast volume and furnace, in smelting ore C theslag solvent is not a blast furnace slag (from a pig iron operation, aswas B. F. I) but is the chrome furnace slag" produced in the operationdescribed in Example 1. The rounds are: 2520 lbs. of coke, 8450 lbs. ofthe chrome furnace slag, and 8450-1bs. of chrome ore C. The blasttemperature is 1400 F., and at each 30-minute flush there is tapped23,300 lbs. of slag solution" showing an analysis: 08.0 18.7%; MgO 11.3;A1203 16.4; Sl0z 27.0; CrzOs 25.2; FeO 1.4; and S 0.62. Metal (mostlyiron) accumulates below the slag bath at thev rate of 4100 lbs./hour,and analyzes 4.1% chromium, in-

'dicating a loss of chromium of 2.03% in the Altos-11.7;

course of converting the ore into a slag solution.

In the present illustration the slag solvent is a.

material which has been produced in the second step of the completeprocess, and illustrates a recycling of the carrier solvent in contrastto the employment of a blast furnace slag taken from another furnaceoperation. The value and cost of these slags is so small that theselection of a solvent slag depends upon location, availability,transportation, and handling, rather than on any metallurgicalconsideration.

Whereas it is desirable and essential that the basicity of the solutionproduced be acid, to suppress the formation of calcium chromite, it isnot necessary in every case that the added solvent Example 4 The ironblast furnace slag (B. F. 11), used as the diluting solvent, is as basicas is generally encountered in practice, having the analysis Ca46.4%;Mg0-6.5; Alma-16.1; SiO2- 29.7; Fe00.3;. MnO-OA; 8-1.3. This carriersolvent has a basicity materially greater than unity, viz.,CaO+MgO=52.9%, A1203+Si02= 45.8%, or a basicity ratio of 1.15.

The furnace of the previous illustration is I have found that slagsolutions containing as 8 much as 15% to 25% CrzOa are handled in thebosh and hearth of the blast furnace without noticeable diflicultiesfrom the standpoints of infusibility and viscosity, as in ordinary ironfurnace slags. In the second step of the process no difficulty insmelting ores is encountered, the reduction of even thesedilutesolutions of CEO:

readily being carried on to substantial completion with a recovery ofbetterthan 90% of the chromium. Whenever the relative proportion of thegangue oxides MgO, A1203 and Si02, originally present in the chrome ore,permits, I prefer to use a more concentrated chrome solution, with CrzOsin excess of 30% or 35% When the Crg0a content is as great as or above,dif ficulties in smelting arise comparable to the difficulties insmelting an untreated and undiluted ore. I have found that whenever asufliciently high blast temperature is available, most chrome ores maybe smelted directly, as described in the hereinbefore mentionedcopending application. In almost all usual circumstances the preliminaryfurnacing of a chrome ore with the solvent, in order to dilute the sameinto a fused acid mixture with Si02, A1203, MgO and 02.0, effects asufficient improvement in its smeltability so that the double furnacingeffects a material improvement in the over-all economy of the process,as well as in the quality of the ultimate product.

It should be understood that Whereas I prefer to introduce the dilutingoxides S102, A1203, CaO and MgO into the blast furnace in the form of aprefused slag, it is possible to carry out the first step of the processby adding the proper amounts of SiOz, A1203, MgO and CaO in merelyphysical admixture: for example, the S102 and A120: y

be added as an argillaceous shale, and the Ca0 and MgO as dolomite or asdolomitic limestone.

When the prefused slag is added, the constituents of the solvent meltpromptly and flow more rapidly than when the several basic and acidoxide diluents are added in the form of discrete mineral lumps, in whichlatter case some delay is occasioned in realizing contact between theconstituent-oxides to form the liquid solvent. In most localities wherethe furnacing of chrome ores by my process is convenient, an abundanceof blast furnace slag is available -and cheap.-

Whenever the composition of the preformed slag to be employed as solventis not adapted to the production of a desirable solution, itscomposition can be adjusted by the addition of acid oxides or of basicoxides, as the case requires, to the furnace charge in addition to thecoke, chrome ore and slag.

I prefer for reasons of cost to carry out both steps of the process insuitable blast furnaces. I can use two blast furnaces, one of which isdevoted to one step, the other to the other; or,

when plant facilities are limited, I can carry out thetwo steps of theprocess in the same furnace but at different times. 4

It is pointed out that in carrying out the first step of this processthe metallurgical objective is the dilution of the chrome ore to producean acid slag solution of diminished Cr2Oa content. This means that theblast furnace is run as a melting furnace of high thermal efficiencyemploying a cheap fuel. The fact-that the blast furnace so used is anotably eflicient reducing furnace does not dictate its employment forthe first step. on the contrary, in carrying out the first step of myprocess I operate the blast furnace under as restricted reducingconditions as I am able to realize without destroying its effectivenessas a melting furnace, in order to maintain at a minimum the amount ofmetallization of the reducible oxides contained in the charge. Themethods of obtaining diminished reduction in the bosh and hearth of ablast furnace I have described in my copending application Serial No.234,847., filed October 13, 1938, entitled "Blast furnace treatment oflow grade manganese-iron ore (now Patent No. 2,265,863). While in theprocess of that application the reduction being suppressed is reductionof MnO, and not CrzOa as in the present method, the operative methodstherein described I have found to be equally effective in minimizing thereduction of the crzoa. and whenever these methods are applicable Ispecifically include them as ancillary features of the presentinvention.

This application is a division of my application Serial No. 260,866,filed March 9, 1939, now Patent No. 2,265,865.

I claim:

l. Two-step process for the production of pig iron and ferro-chromiumfrom a low-grade chrome ore containing, together with gangue material,a. larger ratio of iron to chromium than is desired in the finalferro-chromium product, which comprises as a first step charging into ablast furnace the low-grade chrome ore, solid carbonaceous fuel and amineral solvent for the chromium oxide content of said ore, said mineralsolvent consisting predominantly of the common refractory oxides A1203,S102, 09.0 and MgOand low in reducible oxides such as the oxides of ironI and manganese, the ingredients of said mineral solvent being soproportioned as to' yield with the gangue material of said ore an acidprimary slag, operating the blast furnace under minimized reducingconditions adapted to depress metallization of chromium oxide whileproducing molten iron and a molten acid primary slag solution ofchromium oxide in which the concentration of chromium oxide is lowerthan was the concen-.

. to produce ferro-chromium and a final slag poor in recoverable metalvalues.

2. The process defined in claim 1, wherein the mineral solvent employedin the first step is a composition exhibiting a minimum viscosity in thetemperature range 26003000 F.

3. The process defined in claim 1, wherein the mineral solvent employedin the first step consists mainly of a slag fromthe blast furnacesmelting of iron ore.

4. The process defined in claim 1, wherein the mineral solvent employedin the first step consists essentially of recycled final slag from the10 second steppf the process.

PERCY H. ROYS'I'ER.

